Percentages of individuals survivingFig. 3.Percentages of individuals captured each month surviving in subsequent months. The graph shows differential survival according to time of birth. Individuals born in autumn seem to have a longer life expectancy. The numbers on the lines refer to months of first capture.
Fig. 3.Percentages of individuals captured each month surviving in subsequent months. The graph shows differential survival according to time of birth. Individuals born in autumn seem to have a longer life expectancy. The numbers on the lines refer to months of first capture.
A study of the age groups in each month's population revealed adifferential survival based on the season of birth. Blair (1948:405) found that chances of survival inMicrotus pennsylvanicuswere approximately equal throughout the year. In the present populations ofM. ochrogaster, however, voles born in October, November, December and January tended to live longer than those born in other months (Fig. 3). Presumably these animals, born in autumn and early winter, were more vigorous than their older competitors and were therefore better able to survive the shrinking habitat of winter. Their continued survival after large numbers of younger voles had been added to the population probably was permitted by the expanding habitat of spring and summer. The percentage of the population surviving the winter of 1951-1952 was approximately double the percentage surviving the winter of 1950-1951. This difference seemed to be due to the smaller population entering the winter of 1951-1952 rather than any major difference in the environmental resistance.
As a consequence of the differential survival, most of the breeding population in the spring was made up of animals born the previous October and November.Fig. 4shows that in February, when the percentage of breeding females ordinarily began to rise, 51.6 per cent of the population was born in the previous October and November. Voles born in these two months continued to form a large part of the population through March (45.1 per cent), April (38.5 per cent), May (23.9 per cent), June (18.7 per cent) and July (16.2 per cent) (Fig. 4). These percentages suggest that the habitat conditions in October and November were probably important in determining the population level for at least the first half of the next year.
Differential survival of volesFig. 4.Differential survival of voles according to month when first caught. Each column represents the percentage of the monthly sample first caught in each of the preceding months. Those voles caught first in October and November survived longer than those first caught in other months. Relatively few individuals remained in the population as long as one year.
Fig. 4.Differential survival of voles according to month when first caught. Each column represents the percentage of the monthly sample first caught in each of the preceding months. Those voles caught first in October and November survived longer than those first caught in other months. Relatively few individuals remained in the population as long as one year.
Population densities were ascertained on the study areas by means of the live-trapping program. Blair (1948:396) stated that almost all small mammals old enough to leave the nest (except shrews and moles) are captured by live-trapping. My experience, and that of other workers on the Reservation, requires modification of such a statement. The distance between traps is an important factor in determining the efficiency of live-trapping. As mentioned earlier, when House Field and Quarry Field were trapped out at the conclusion of the live-trapping program no unmarked voles were taken. This showed that the 30 foot interval between traps was short enough to cover the area as far asMicrotuswas concerned. The fact that unmarked adults were caught almost entirely in marginal traps is additional evidence. On the other hand, the Fitch traps were 50 feet apart and voles seemed to have lived within the grid for several months before being captured. Fitch (1954:39) has shown that some kinds of small mammals are missed in a live-trapping program because of variation in bait acceptance, both seasonal and specific.
A few individuals, missed in a trapping period, were captured again in subsequent months. These voles were assumed to have been present during the month in which they were not caught. The area actually trapped each month was estimated by a modification of the method proposed by Stickel (1946:153). The average maximum move was calculated each month and a strip one half the average maximum move in width was added to each side of the study area actually covered by traps. The study plots were bounded in part by gravel roads and forest edge acting as barriers, and for these parts no marginal strip wasadded. Trap lines on the opposite sides of these roads rarely caught marked voles that had crossed in either direction. It is perhaps advisable to say here that the size of House Field and Quarry Fieldstudy plots (0.56 acres) was too small for best results in estimating population levels (Blair, 1941:149). In the computations of population levels the data for males and females were combined, because no significant difference between the average maximum move of the sexes was apparent.
Fluctuations of the populations were graphed in terms of individuals per acre (Fig. 5). The variation was great in the 30 month period for which data were available, and was both chronological and topographical. The lowest density recorded was 25.2 individuals per acre and the highest density was 145.8 individuals per acre. The weight varied from a low of 847 grams per acre to a high of 5275 grams per acre.
Variations in density of volesFig. 5.Variations in density of voles from three populations, as shown by live-trapping, and the mean density of these populations. Juveniles are not represented in their true numbers since many voles were caught first as subadults. The samples from the Fitch trap line were incomplete due to the wide spacing of the traps.
Fig. 5.Variations in density of voles from three populations, as shown by live-trapping, and the mean density of these populations. Juveniles are not represented in their true numbers since many voles were caught first as subadults. The samples from the Fitch trap line were incomplete due to the wide spacing of the traps.
There are few records of density ofM. ochrogasterin the literature. Brumwell (1951:213) found nine individuals per acre in a prairie on the Fort Leavenworth Military Reservation and Wooster (1939:515) reported 38.5 individuals per acre forM. o. haydeniin a mixed prairie in west-central Kansas. High densities forM. pennsylvanicusreported in the literature include 29.8 individuals per acre (Blair, 1948:404), 118 individuals per acre (Bole, 1939:69), 160-230 individuals per acre (Hamilton, 1937b:781) and 67 individuals per acre (Townsend, 1935:97).
Because the study period included one period of unusually high rainfall and one year of unusually low rainfall, the normal pattern of seasonal variation of population density was obscured. An examination of the data suggested, however, that the greatest densities were reached in October and November with a second high point in the April-May-June period. These high points generally followed the periods of high levels of breeding activity (Fig. 8). The autumn rise in population may have been due, in part, to the addition of spring and early summer litters to the breeding population, but the rise occurred too late in the year to be explained by that alone. Another factor may have been the spurt in growth of grasses occurring in Kansas in early autumn, in September and October. There was a seeming correlation between high rainfall with rapid growth of grasses and reproductive activity, and, secondarily with high population densities of voles. These relationships are discussed in connection with reproduction. Lowest annual densities were found to occur in January when there is but little breeding activity and when rainfall is low and plant growth has ceased.
Marked deviation from the usual seasonal trends accompanied flood and drought. In the flood of July, 1951, although the study areas were not inundated, the ground was saturated to the extent that every footprint at once became a puddle. Immediately after the floods, on all three areas studied, populations were found to have been drastically reduced. The effect was most severe on the population of House Field, the lowest area studied, and the recovery of the population there was much slower than that of those on the other study areas (Fig. 5). Newborn voles were killed by the saturated condition of the ground in which they lay. The more precocious young ofSigmodon hispidussurvived wetting better. They thus acquired an advantage in the competitive relationship between cotton rats and voles. These relationships are discussed more fully in the section on mammalian associates ofMicrotus.
Adverse effects of heavy rainfall on populations of small mammals have been reported by Blair (1939) and others. Goodpastor and Hoffmeister (1952:370) reported that inundation sharply reducedpopulations ofM. ochrogasterfor a year after flooding but that the area was then reoccupied by a large population of voles. Such a reoccupation may have begun on the areas of this study in the spring of 1952 when the upward trend of the population was abruptly reversed by drought. While cotton rats were abundant their competition may have been an important factor in depressing population levels of voles. The population of voles began to rise only after the population of cotton rats had decreased (Fig. 19).
In the unusually dry summer of 1952, there was a marked decline of population levels beginning in June and continuing to August when my field work was terminated. Dr. Fitch (1953,in litt.) informed me that the decline continued through the winter of 1952-53 and into the summer of 1953, until daily catches ofMicrotuson the Reservation were reduced to 2-10 per cent of the number caught on the same trap lines in the summer of 1951. The drought seemed to affect population levels by inhibiting reproduction, as described elsewhere in this report. A similar sensitivity to drought was reported by Wooster (1935:352) who foundM. o. haydenidecreased more than any other species of small mammal after the great drought of the thirties.
No evidence of cycles inM. ochrogasterwas observed in this investigation. All of the fluctuations noted were adequately explained as resulting from the direct effects of weather or from its indirect effect in determining the kinds and amounts of vegetation available as food and shelter.
The differences in densities supported by the various habitats were discussed earlier in connection with the analysis of habitats.
Home ranges were calculated for individual voles according to the method described by Blair (1940:149-150). The term, home range, is used as defined by Burt (1943:350-351). Only those voles captured at least four times were used for the home range studies. Individuals which included the edge of the trap grid in their range were excluded unless a barrier existed (see description of habitat) confining the seeming range to the study area.
The validity of home range calculations has been challenged (Hayne, 1950:39) and special methods of determining home range have been advocated by a number of authors. The ranges calculated in this study are assumed to approximate the actual areas used by individuals and are considered useful for comparison with other ranges calculated by similar methods, but no claim to exactness is intended. It is obvious, for instance, that many plotted ranges contain so-called blank areas which, at times, are not actually used by any vole (Elton, 1949:8; Mohr, 1943:553). Studies of the movements of mammals on a more detailed scale, perhaps by live-traps set at shorter intervals and moved frequently, are needed to increase our understanding of home range.
In order to test the reliability of the range calculated, an examination of the relationship between the size of the seeming range and the number of captures was made. For the first three months, trapping on House Field was done with a 20 foot grid and throughout the remainder of the study a 30 foot grid was used. The effect of these different spacings on the size of the seeming home range was also investigated. Hayne (1950:38) found that an increase in the distance between traps caused an increase in the size of the seeminghome range, but in my study the increased interval between traps was not accompanied by any change in the sizes of the calculated ranges.
The number of captures, above the minimum of four, did not seem to be a factor in determining the size of the calculated monthly range. A seeming relationship was observed between the number of times an individual was trapped and the total area used during the entire time the vole was trapped. Closer examination revealed that the most important factor was the length of time over which the vole's captures extended.Table 2shows the progressive increase in sizes of the mean range of animals taken over periods of time from one month to ten months.
Table 2. Relationship Between Home Range Size and Length of Time on the Study Area
No. months on area12345678910Mean range in acres.09.09.10.14.13.17.22.22.26.24
Nothing concerning the home range ofMicrotus ochrogasterwas found in the literature. Several workers, including Blair (1940) and Hamilton (1937c), have studied the home range ofM. pennsylvanicus. Blair (1940:153) reported a larger range for males than for females in all habitats and in all seasons represented in his sample. InM. ochrogaster, however, I found that the mean monthly range for both sexes was 0.09 of an acre. Blair (loc. cit.) reported no individuals with a range so small as that mean, but Hamilton (op. cit.:261) mentioned two voles with ranges of less than 1200 square feet. The mean total range used by an individual during the entire time it was being trapped showed a slight difference between the sexes. Males used an average of 0.14 of an acre whereas females used an average of but 0.12 of an acre. This suggested that, as inM. pennsylvanicus(Hamilton,loc. cit.), males tended to wander more than females and to shift their home range more often.
The largest monthly range recorded was 0.28 of an acre used by a female in March, 1951, and calculated on the basis of four captures. The largest monthly range of a male was 0.25 of an acre for a vole caught eight times in November, 1950. The smallest monthly range was 0.02 of an acre; several individuals of both sexes were restricted to areas of this size. Juveniles, not included in the home range study, were usually restricted to 0.01 or, at most, 0.02 of an acre. Seasonal differences in the sizes of home ranges were not significant. However, the voles caught in the winter often enough to be used for home range studies were too few for a thorough study of seasonal variation in the size of home ranges.
One female was captured 22 times in the seven-month period of October, 1950, to April, 1951. She used an area of 0.83 of an acre, but this actually comprised two separate ranges. From October, 1950, through December, 1950, she was taken 17 times within an area of 0.12 of an acre; and from January, 1951, to April, 1951, she was taken five times within an area of 0.15 of an acre. The largest area assumed to represent one range of a female was 0.38 of an acre, recorded on the basis of six captures in three months. The largest area encompassed by the record of an individual male was 0.41 of an acre. He, too, shifted his range, being taken five times on an area of 0.07 of an acre and twice, two months later, on an area of 0.09 of an acre. Presumably,the remainder of his calculated total range was used but little, or not at all. The largest single range of a male was 0.36 of an acre, calculated on the basis of 18 captures in seven months. The smallest total range for both sexes was 0.02 of an acre.
Many voles shifted their home range and a few did so abruptly. The large range of a female vole, described above and plotted inFig. 6, indicated an abrupt shift from one home range to another. More common is a gradual shift as indicated by the range of the male shown inFig. 7. Large parts of each monthly range of this vole overlapped the area used in other months but his center of activity shifted from month to month.
Map with cross-hatched areas showing the range of vole #20Fig. 6.Map with cross-hatched areas showing the range of vole #20 (female). Dots show actual points of capture at permanent trap stations 30 feet apart. Vertical lines mark area in which vole was taken 17 times in October and November, 1950. Horizontal lines mark area in which vole was taken five times in March and April, 1951. This vole was not captured in December and January.
Fig. 6.Map with cross-hatched areas showing the range of vole #20 (female). Dots show actual points of capture at permanent trap stations 30 feet apart. Vertical lines mark area in which vole was taken 17 times in October and November, 1950. Horizontal lines mark area in which vole was taken five times in March and April, 1951. This vole was not captured in December and January.
Map showing range of vole #52Fig. 7.Map showing range of vole #52 (male) with seeming shifts in its center of activity. Dots show actual points of capture at permanent trap stations 30 feet apart. Solid line encloses points of six captures in October and November, 1950. Broken line encloses points of five captures in February and March, 1951. Dotted line encloses points of nine captures in April, May and June, 1951.
Fig. 7.Map showing range of vole #52 (male) with seeming shifts in its center of activity. Dots show actual points of capture at permanent trap stations 30 feet apart. Solid line encloses points of six captures in October and November, 1950. Broken line encloses points of five captures in February and March, 1951. Dotted line encloses points of nine captures in April, May and June, 1951.
That home ranges overlapped was demonstrated by frequent capture of two or more individuals together in the same trap. No territoriality has been reported in any species ofMicrotus, to my knowledge, and my voles showed no objection to sharing their range. Voles taken from the field into the laboratory lived together in pairs or larger groups without much friction.
Definable systems of runways and home ranges were not coextensive. Runway systems tended to merge, as described later in this report, and relationships between them and home range were not apparent. Home ranges had no characteristic shape.
Reproductive activity might have been measured in a number of ways. Three indicators were tested: the percentage of females gravid or lactating, the percentage of juveniles in the month following the sampling period, and the percentage of females with a vaginal orifice in the sampling period. The condition of vagina proved to be most useful. Whether or not there is a vaginal cycle inMicrotusis uncertain. Bodenheimer and Sulman (1946:255-256) found no evidence of such a cycle, nor did I in my work with laboratory animals at Lawrence. How much the artificial environment of the laboratory affected these findings is unknown. The presence of an orifice seemed to indicate sexual activity (Hamilton, 1941:9). The percentage of gravid females in the population could not be determined accurately by a live-trapping study and was not useful in this investigation. The percentage of juveniles trapped in the month following the sampling period tended to follow the curve of the percentage of adult females with a vaginal orifice. The ratio of trapped juveniles to adults trapped was a poor indicator of reproductive activity. Juveniles were caught in relatively small numbers because of their restricted movements, and no way to determine prenatal and juvenal mortality was available.
Reproductive activity continues throughout the year. Within the thirty-month period for which data were obtained, December and January showed the lowest percentages of females with vaginal orifices (Fig. 8). The other months all showed higher levels of reproductive activity with a slight peak in the August-September-October period in both 1950 and 1951. In the species ofMicrotusthat are found in the United States, such summer peaks of breeding seem to be the rule (Blair, 1940:151; Gunderson, 1950:17; Hamilton, 1937b:785). Jameson (1947:147) worked in the same county where my field study was made and found that the high point of reproduction was in March, although his samples were too small to be reliable. The peak of reproductive activity slightly preceded the highest level of population density in each year (Fig. 8).
Variations in density and reproductive rate of volesFig. 8.Variations in density and reproductive rate of voles, with variation in monthly precipitation. Abnormally low rainfall in 1952 caused a decrease in breeding activity and eventually in the numbers of voles. The solid line indicates the number of voles per acre, the broken line the percentage of females with a vaginal orifice and the dotted line the inches of rainfall.
Fig. 8.Variations in density and reproductive rate of voles, with variation in monthly precipitation. Abnormally low rainfall in 1952 caused a decrease in breeding activity and eventually in the numbers of voles. The solid line indicates the number of voles per acre, the broken line the percentage of females with a vaginal orifice and the dotted line the inches of rainfall.
A marked reduction in the percentage of females having vaginal orifices was observed in the unusually dry summer of 1952. The rate of reproduction was found to be positively correlated with rainfall (Fig. 9). Correlation coefficients were higher in each case when the amount of rainfall in the month preceding each sampling period was used instead of that in the month of the sample. This suggested that the rainfall exerted its influence indirectly through its effect on plant growth. Bailey (1924:530) reported that a reduction in either the quantity or quality of food had a depressing effect on reproduction. Drought, such as occurred in 1952, would certainly have a depressing effect on both. The critical factor seems to be the supply of new, actively growing shoots available to the voles for food rather than the total amount of vegetation. As far as could be determined from the small sample of males examined, their fecundity was not affected by rainfall. Some decrease in the percentage of males that were fecundwas noted in the winter and was reported also by Jameson (1947:145) but most of the males in any sample were fecund. Thus any depression in the reproductive rate was due to loss of fecundity by females. This was in agreement with reports in the literature on the subject (Baker and Ransom, 1932a:320; 1932b:43).
The correlation coefficient between rainfall and the percentage of adult females with a vaginal orifice was 0.53. This was considered to be surprisingly high in view of the expected effects on the breeding rate of temperature, seasonal diet variations and whatever rhythms were inherent in the voles. When only the summer months were considered the correlation coefficient between rainfall and the percentage of adult females with a vaginal orifice was 0.84. This indicated that, during the season when breeding was at its height, rainfall was a factor in determining the rate of reproduction and when rainfall was scarce, as in the summer of 1952, it seemed to be a limiting factor (Fig. 9).
Comparison between monthly rainfall and reproductive rate of voles in summerFig. 9.Comparison between monthly rainfall and reproductive rate of voles in summer. The dry summer of 1952 caused a notable decrease in reproductive activity. The correlation coefficient between rainfall and the percentage of females with a vaginal orifice was 0.84.
Fig. 9.Comparison between monthly rainfall and reproductive rate of voles in summer. The dry summer of 1952 caused a notable decrease in reproductive activity. The correlation coefficient between rainfall and the percentage of females with a vaginal orifice was 0.84.
Of the total captures 20.6 per cent involved more than one individual. When the distribution of these multiple captures wasgraphed for the period of study, a high correlation between the percentage of captures that were multiple and the percentage of females with a vaginal orifice (r = 0.70) was found. An even higher correlation (r = 0.76) was observed between the percentage of captures that were multiple and the population density. The higher percentage of multiple captures may have been largely a result of fewer available traps per individual on the area and thus only indirectly related to the rate of reproduction.
Of the multiple captures, 66 per cent involved both sexes. The correlation coefficient between the percentage of captures involving both sexes and the level of reproductive activity was 0.58. Among those pairs of individuals caught together more than once, 61 per cent were composed of both sexes. Among those pairs taken together three or more times 76 per cent were male and female and among those pairs taken together four or more times 80 per cent were male and female. When adult voles stayed together any length of time their relationship usually appeared to be connected with sex. Family groups were also noted, as pairs were often trapped which seemed to be mother and offspring. A lactating female would sometimes enter a trap even after it had been sprung by a juvenile, presumably her offspring, or a juvenal vole would enter a trap after its mother had been captured. Such family groups persisted only until the young voles had been weaned.
The youngest female known to be gravid was 26 days old and weighed 28 grams. During summer most of the females were gravid before they were six weeks old, although females born in October and after were often more than 15 weeks old before they became gravid. The youngest male known to be fecund was approximately six weeks old. Male fecundity was determined as described by Jameson (1950). Difference in the age of attainment of sexual maturity serves to reduce the mating of litter mates (Hamilton, 1941:7) and has been noticed in various species of the genusMicrotusby several authors (Bailey, 1924:529; Hatfield, 1935:264; Hamilton,loc. cit.; Leslie and Ransom, 1940:32).
For 35 females, each of which was caught at least once each month for ten consecutive months or longer, the mean number of litters per year was 4.07. Certain of the more productive members of the group produced 11 litters in 16 months.M. ochrogasterseems to be less prolific thanM. pennsylvanicus. Bailey (1924:528) reported that one female meadow vole delivered 17 litters in 12 months. Hamilton (1941:14) considered 17 litters per year to be the maximum and stated that in years when the vole population was low the females produced an average of five to six litters per year. In "mouse years" the average rose to eight to ten litters per year. During this study several females delivered two or more litters in rapid succession. This was noted more frequently in spring and early summer than in other parts of the year. Those females which produced two or three litters in rapid succession in spring and early summer often did not litter again until fall. Post-parous copulation has been observed inM. pennsylvanicusby Bailey (1924:528) and Hamilton (1940:429; 1949:259) and probably occurs also inM. ochrogaster.
The gestation period was approximately 21 days, the same as reported forM. pennsylvanicus(Bailey,loc. cit.; Hamilton, 1941:13) andM. californicus(Hatfield, 1935:264). A more precise study of the breeding habits ofM. ochrogasterfailed to materialize when the voles refused to breed in captivity. Fisher (1945:437) also reported thatM. ochrogasterfailed to breed in captivity althoughM. pennsylvanicus(Bailey, 1924) andM. californicus(Hatfield, 1935) reproduced readily in the laboratory.
In the course of this study 65 litters were observed. The mean number of young per litter was 3.18 ± 0.24 and the median was three (Fig. 10). Three litters contained but one individual and the largest litter contained six individuals. Other investigators have reported the number of young per litter inM. ochrogasteras three or four (Lantz, 1907:18) and 3.4 (1-7) (Jameson, 1947:146).M. pennsylvanicusseems to have larger litters. Although Poiley (1949:317) found the mean size of 416 litters to be only 3.72 ± 0.18, both Bailey (1924:528) and Hamilton (1941:15) found five to be the commonest number of young per litter in that species. Leslie and Ransom (1940:29) reported the average number of live births per litter to be 3.61 in the British vole,M. agrestis. Selle (1928:96) reported the average size of five litters ofM. californicusto be 4.8. Hatfield (1935:265), working with the same species, found that litter size varied directly with the age of the female producing the litter. He reported litters of young females as two to four young per litter and of older females as five to seven young per litter. In thelitters ofM. ochrogasterthat I examined, young females did not have more than three young and usually had but two. However, older females had litters of one, two and three often enough so that no relationship, as described above, was indicated clearly.
Distribution of litter size among 65 litters of volesFig. 10.Distribution of litter size among 65 litters of voles.
Fig. 10.Distribution of litter size among 65 litters of voles.
No seasonal variation in litter size was noted. The mean size of the litters in 1950, 2.68 ± 0.30, was significantly lower than that found in 1951 (3.76 ± 0.20) but neither differed significantly from the mean size of litters in 1952 (3.35 ± 0.66). The lower mean size of litters was in part coincidental with a high population level and the higher mean of the two later years was in part coincidental with a low population level. Since a sharp break in the curve for population density occurred after the flood in July, 1951, the litters were arranged in pre-flood and post-flood categories for study. Pre-flood litters averaged 3.07 ± 0.28 young per litter whereas post-flood litters averaged 3.34 ± 0.48. This difference was not significant. Increase in litter size, if it had actually occurred, might have been a response to the increasing food supply and lower population density after the flood.
A difference in the mean number of young per litter was noted for those litters delivered in traps as compared with those delivered in captivity and the numbers of embryos examined in the uterus. The mean number of embryos per female was higher than the mean number of young per litter delivered in captivity and the mean number of young per litter delivered in traps was lower than in those delivered in captivity. The differences were not statistically significant. In some instances females that delivered young voles in traps may have delivered others prior to entering the trap or the mother or her trapmates may have eaten some of the newborn voles before they were discovered.
The mean weight of 16 newborn (less than one day old) individuals was 2.8 ± 0.36 grams. No other data on the weight of newbornM. ochrogasterwere found in the literature but this mean was close to the 3.0 grams (Bailey, 1924:530) and 2.07 grams (Hamilton, 1937a:504; 1941:10) reported forM. pennsylvanicusand to the 2.7 grams (Selle, 1928:97) and 2.8 grams (Hatfield, 1935:268) reported forM. californicus. No correlation between the weight of the individual newborn vole and the number of voles per litter was observed.
Although the ratio of the average weight of newborn voles to the average weight of an adult female was approximately equal forM. pennsylvanicusandM. ochrogaster, the ratio of the weight of a litter to the average weight of an adult female was larger in the eastern meadow vole because the mean litter size was larger. Perhaps this is related to the more productive habitat in which the eastern meadow vole is ordinarily found.
The mean weight of adult voles during the period of study was 43.78 grams. The females averaged slightly heavier than the males but the overlapping of weights was so extensive that sexual difference in weight could not be affirmed. The difference observed was less in December and January when gravid females were rare, suggesting that the difference was due, at least in part, to pregnancy. Jameson (1947:128) found, for a sample of 50 voles, a mean weight of 44 grams and a range of 38 to 58 grams. The range in the adult voles I studied was much greater, from 25 to 73 grams. In part, this increase in the range of adult weights was due to a much larger sample.
Relationship between rainfall and the mean weightFig. 11.Relationship between rainfall and themean weight of adult malesin summer. The abnormally low rainfall in the summer of 1952 was accompanied by a decrease in mean weight. The solid line represents mean weight and the broken line rainfall. The correlation coefficient between the two was 0.68.
Fig. 11.Relationship between rainfall and themean weight of adult malesin summer. The abnormally low rainfall in the summer of 1952 was accompanied by a decrease in mean weight. The solid line represents mean weight and the broken line rainfall. The correlation coefficient between the two was 0.68.
During the unusually dry summer of 1952, a notable reduction in the mean weight of adults was recorded (Fig. 11). The correlation coefficient between the mean weight of adults and the amount of rainfall for the summer months was 0.68. It seems reasonable toattribute the drop in mean weight to an alteration of plant growth due to decreased rainfall. Some of the reduction in mean weight was due to the loss of weight in older individuals but most of it was due to the failure of voles born in the spring to continue growing.
No data on the growth rate ofM. ochrogasterwere found in the literature. According to the somewhat scanty data from my study, secured from observations of individuals born in the laboratory, young voles gained approximately 0.6 of a gram per day for the first ten days, approximately one gram per day up to an age of one month, and approximately 0.5 of a gram per day from an age of one month until growth ceases. This growth rate was especially variable after the voles reached an age of thirty days. The growth rate approximates those described forM. pennsylvanicus(Hamilton, 1941:12) and forM. californicus(Hatfield, 1935:269; Selle, 1928:97). Although the data were inadequate for a definite statement, I gained the impression that there was no difference between the sexes in growth rate. In general, young voles grow most rapidly in the April-May-June period and least rapidly in mid-winter. Several voles, born in late autumn, stopped growing while still far short of adult size and lived through the winter without gaining weight, then gained as much as 30 per cent after spring arrived (Fig. 12).
Growth rates of two volesFig. 12.Growth rates of two voles selected to show typical growth pattern of voles born late in the year. Growth nearly stops in winter and is resumed in spring.
Fig. 12.Growth rates of two voles selected to show typical growth pattern of voles born late in the year. Growth nearly stops in winter and is resumed in spring.
The recorded life spans of most voles studied were less than one year. No accurate mean life span could be determined. Leslie and Ransom (1940:46), Hamilton (1937a:506) and Fisher (1945:436) also found that most voles lived less than one year. Leslie and Ransom (op. cit.: 47) reported a mean life span of 237.59 ± 10.884 days in voles of a laboratory population. In the present study one female was trapped 624 days after first being captured; another female was trapped 617 days after first being captured; and a male was trapped 611 days after first being captured. The two females were subadultswhen first captured. The male was already an adult when first captured; consequently its life span must have exceeded 650 days. No evidence of any decrease in vigor or fertility was observed to accompany old age.
Of the 45 marked voles snap-trapped in August of 1952, 21 had been captured first as juveniles. The ages of these voles could be estimated within a few days, and the series presented a unique opportunity for studying individual and age variation. Only individuals weighing less than 18 grams when first captured were used, and their ages were estimated according to the growth rate described above. Howell (1924) reported an analysis of individual and age variation in a series of specimens ofMicrotus montanus, and Hall (1926) studied the changes due to growth in skulls ofOtospermophilus grammarus beecheyi. The series of specimens described here differs from those of Hall and Howell, and from any other collection known to me, in the fact that the specimens are of approximately known age and drawn from a wild population.
Unfortunately, this sample was small, and the distribution of the specimens among age groups left much to be desired. No specimens less than one and one-half months old were taken and only a few individuals older than four and one-half months.Table 3shows the age distribution. The small size of the sample and the absence of juveniles were due, partly, to the unusually dry weather in the summer of 1952. The reduction in the rate of reproduction, caused by drought (as described elsewhere in this paper), reduced the populations and the percentage of juveniles to low levels.
Table 3. Distribution Among Age Groups of 21 Voles Used in the Study of Variation Due to Age
Age in months11⁄2221⁄2331⁄2441⁄2612No. of individuals145132311
In the series of voles studied, ten individuals were in the process of molting from subadult to adult pelage. Jameson (1947:131) reported the molt to occur between eight and 12 weeks of age and selected 38 grams as the lower limit of weight of adults. I also found all voles molting to be between eight and 12 weeks old but found none so large as 38 grams without full adult pelage. This may have been, in part, due to the dry weather delaying or inhibiting growth. Because of the small size of the sample and the influence of the unusual weather conditions, no conclusions concerning normal molting were drawn from the data described below. They are presented only as a description of a small sample drawn from a single population at one time.Table 4summarizes these data.
Table 4. Mean Sizes and Ages of Voles Molting from Subadult to Adult Pelage
WeightBody length minus tailCondylo-basilar lengthAgeSix males32.67 gms.106.16 mm.23.78 mm.9.67 wks.(30-36)(96-116)(23.2-24.4)(8-12)Four females29.0 gms.100.25 mm.23.45 mm.10.5 wks.(28-30)(98-102)(23.5-23.8)(8-12)Ten voles31.2 gms.103.8 mm.23.73 mm.10.0 wks.(28-36)(96-116)(23.2-24.4)(8-12)
The mean age of the ten voles molting was ten weeks (8-12). Six malesaveraged 9.67 weeks, almost a week younger than four females, who averaged 10.5 weeks. The difference in age at time of molting between the sexes was not significant. Differences between the sexes in other characteristics to be described also lacked significance. Mean weights at the time of molting were: males, 32.67 gms. (30-36); females, 29.0 gms. (28-30); and all individuals, 31.2 gms. (28-36). Because a piece of the tail of each vole had been removed in marking, the total length of the voles could not be determined. Body length, excluding tail, was used. Howell (1924:986) found this measurement subject to less individual variation than total length and thought body length was probably a better indicator of age. Mean body length at the time of molting was 103.8 mm. (96-116). Males averaged longer than females and were also more variable. The mean body length of males was 106.16 mm. (96-116) and that of females was 100.25 mm. (98-102).
Of the subadults showing no signs of molting, none was above the mean age of molting. Twenty-five per cent of them were longer and heavier than the mean length and weight of those that were molting. Of the 20 adults in the series, one was below the mean weight of molting and one was shorter than the mean length of molting.
When Howell (op. cit.:1014) studied skulls ofMicrotus montanushe found that the condylobasilar length was the most satisfactory means for arranging his series of specimens according to their age. When the skulls of my series were arranged according to their age (as determined from trapping records) the graph of the condylobasilar lengths showed a clear, though not perfect, relationship to age (Fig. 13). No separation of sexes was made because the sample did not permit it. InFig. 13graphs of weight, as determined in the field, and of length (excluding tail) also were included because they are the most easily measured characters of live voles. The graphs indicate individual variation in these characters which limits their usefulness in determining age.